PRIVATE LIVES ** out of **** MUSIC BOX THEATRE Are you in the mood for a bit of bubbly? Champagne is always flowing in the world of Noel Coward and it's on tap at this modest revival of Private Lives. The show has been on Broadway eight times now, starting in 1931, with Coward and Gertrude Lawrence followed by the likes of Tallulah Bankhead, Brian Bedford, Maggie Smith, Richard Burton and Elizabeth Taylor (and John Cullum), Joan Collins and most recently in 2002 with Alan Rickman and Lindsay Duncan in a production that notably went for a deeper emotional truth underlying the flippant one-liners